A number of background art identification or communication systems exist, but all have various limitations in use where more than one remote device is to be identified, particularly where such identification or communication is to occur with more than one device within a relatively short space of time or substantially simultaneously.
In some prior art systems, such as Voles (U.S. Pat. No. 4,862,176), Nard (U.S. Pat. No. 4,297,700), Kratzer (U.S. Pat. No. 4,095,226), Maier (U.S. Pat. No. 4,034,371) a single frequency is used for both transmission and reception of coded signals. Thus, if two devices communicate together, there will be interference.
In some other prior art, such as Alpers (U.S. Pat. No. 4,350,982), an interrogation pulse of known frequency is intercepted by the transponder, delayed and rebroadcast. The carrier frequency of the rebroadcast is the same as the original interrogation. Thus, it is not desirable to use a system for two or more remote devices at the same time.
In still another system, Rabow (U.S. Pat. No. 4,206,462), Schrader (U.S. Pat. No. 3,750,168) uses a single frequency interrogating signal. A correlation sequence is detected by the transponder and rebroadcast at a different single frequency in which a sub carrier is superimposed on the interrogating signal for synchronism between interrogator and remote device. Such systems are undesirable where synchronism is not available or difficult to achieve and particularly where more than one remote device is to be communicated or identified at the one time.
Other systems, such as Goyer (U.S. Pat. No. 3,887,916), Yost (U.S. Pat. No. 4,006,477), Cleeton (U.S. Pat. No. 3,945,006) utilize frequency-shift keying and transmit by selecting one of two carrier frequencies when the correlation code is 0 or 1. All transponders of this system use the same frequency translated two carrier frequencies. The frequency transmitted by the transponder is controlled by the interrogator and relies on the interrogator having precise knowledge of the signal being transmitted. Again, such a system is not desirable where a number of remote devices of unknown identify or origin are to be identified or communicated with.
Thus, in summary, in the prior art noted above, the frequency used for communication between interrogator and remote device is always known and/or controlled by the interrogator. Such systems do not enable communication or identification with relatively unknown nor a relative large number of remote devices, nor in a substantially simultaneous manner, nor without substantial interference from other transmitting remote devices.
There is a long felt need in many industries, such as the communication, smart card and airline industries, to provide a means whereby two or more transmitters can be interrogated at the same time and where such interrogation leads to identification of and/or communication with each remote device.